how to transfer movies from mini Dv tapes to computer?

Assuming your camera will play them back and you have a suitable lead to connect to your video card/PC - this may be a fire wire lead which will need a special card in the PC.

You can capture the video with Movie maker a free basic editing system from microsoft.

If your camera doesn't have software to do it then you will need a Video capture card. Then you can connect the camera to the PC like you would to watch your videos. Using the software that comes with your capture card you can record the video onto the PC as it plays back from the camera.